1. Field of the Invention
The present invention relates to solid-state imaging devices, methods of driving the solid-state imaging devices, and electronic systems.
2. Description of the Related Art
In a general solid-state imaging device, when electric charges generated by photoelectric conversion are read, the charges are accumulated in an impurity-diffusion region called a floating-diffusion region, or the charges are transferred from a light receiving section to the impurity-diffusion region so that the charges are converted into a voltage in that impurity-diffusion region. A pixel in such a kind of solid-state imaging device generally has a configuration including a reset transistor which resets the floating-diffusion region (hereinafter referred to as an “FD section”) to have a predetermined potential.
In a pixel configuration having a reset transistor, the reset transistor is set to be an on state, and the FD section is initialized to have a fixed voltage Vdd. After that, the reset transistor is changed to an off state to change the FD section to a floating state. The electric charges are stored or transferred to the initialized FD section so that the output voltage produced by charge/voltage conversion in the FD section is obtained. In the reset operation, it is noted that roughly two kinds of noise occur when sampling the initialization voltage of the voltage Vdd in the floating state.
One of the two kinds of noise is a thermal noise (kTC noise), which is dependent on a capacitance of the FD section, and is generated randomly for each reset operation. The other one is noise generated by voltage fluctuations caused by a resistance component of a wiring line at the time of thermal noise and current consumption of the voltage Vdd. Depending on timing of the reset operation, different values are sampled. For the two kinds of noise, it is noted that in a general CMOS-image-sensor operation, noise can be eliminated substantially completely by a noise elimination method called correlated double sampling.
In the correlated-double-sampling processing, a voltage Vo_rst of an FD section, sampled by the reset operation is read, and immediately after that, a stored charge is transferred from a light receiving section to the FD section to be read as a signal voltage Vo_sig. Here, noise by the reset operation is held in the FD section, and thus same noise is overlaid on Vo_rst and Vo_sig. Accordingly, by calculating Vo_sig-Vo_rst, it is possible to obtain an output of the stored charge from which noise caused by a reset operation has been eliminated.
FIG. 28 is a timing waveform chart in the case of an example of driving in which noise is eliminated by correlated double sampling. FIG. 28 shows a selection pulse SEL, which selects a pixel, a reset pulse RST, which resets the FD section, and a transfer pulse TRG and a voltage of the FD section (hereinafter sometimes referred to simply as an “FD voltage”), which reads out a signal charge from a light receiving section to the FD section.
In the case of the example of driving, a signal charge is held by the light receiving section. At the time of readout operation, first, the reset pulse RST becomes active so that a voltage of the FD section is set to a reset voltage Vdd. When the reset pulse RST is active, the voltage of the FD section randomly fluctuates by the fluctuations of the voltage Vdd and thermal noise. The value at the moment when the reset pulse RST has become inactive is fixed as a voltage of the FD section.
At this time, assuming that the fixed noise is ΔVn, the voltage of the FD section becomes Vdd+ΔVn. The voltage Vdd+ΔVn is read out as a reset level Vo_rst, and then the transfer pulse TRG becomes active so that the signal charge of the light receiving section is transferred to the FD section. The FD section is floating, and thus a voltage Vsig for the signal charge is added to the above-described reset level Vdd+ΔVn, resulting in Vdd+ΔVn+Vsig.
The voltage of the FD section at this time Vdd+ΔVn+Vsig is read out as a signal level Vo_sig. The difference with the above-described reset level Vo_rst (=Vdd+ΔVn) is obtained, and a final output Vout becomes as follows, thereby canceling the reset noise ΔVn.
                    Vout        =                ⁢                              (                          Vdd              +                              Δ                ⁢                                                                  ⁢                Vn                            +              Vsig                        )                    -                                                ⁢                  (                      Vdd            +                          Δ              ⁢                                                          ⁢              Vn                                )                                        =                ⁢        Vsig            
However, readout noise other than the reset noise Vo_rst, for example, a so-called 1/f noise, which occurs in an output circuit (an amplification transistor of a source follower circuit, etc.,), is noticeable in a low-frequency band. Accordingly, for reading out of the reset level Vo_rst, if not executed immediately before the readout of the signal level Vo_sig, noise of a low-frequency band is overlaid on the output, and thus it is difficult to obtain advantages of correlated double sampling, thereby resulting in image-quality deterioration.
For this reason, in a solid-state imaging device performing global exposure operation (batch exposure) in which all the pixels are subjected to photoelectric conversion in a same exposure period, a drive method in which after the signal level is read out, the reset operation is performed again to read out the reset level (for example, refer to Japanese Unexamined Patent Application Publication No. 2007-074435). By the global exposure, photoelectric conversion is performed for all the pixels in a same exposure period so that an image without deformation can be obtained from a subject with motion.
Such a drive method is employed in an image sensor accumulating photo-converted electric charges in the FD section directly, for example a solid-state imaging device using an organic photoelectric conversion layer as a light receiving section in addition to a solid-state imaging device performing global exposure operation.
Specifically, in the case of reading out in a state in which the signal charge is held by the FD section, or the signal charge is stored in the FD section, the order of driving becomes the order as shown in FIG. 29. That is to say, after the signal level is read out, a reset operation is performed to obtain the reset level.
To describe more in detail, first, the FD section is reset before the signal charge is transferred to the FD section, or the signal charge is stored. At this time, noise ΔVn′ is overlaid on the rest voltage Vdd.
Electric charges of all the pixels are transferred simultaneously or are directly stored in the FD section during an exposure period so that a signal charge Vsig is added to the voltage of the FD section. Thus, at the point in time of the readout operation, the signal level is already held as Vdd+ΔVn′+Vsig.
In the readout operation, first, the signal level is read out. After that, the reset operation is performed again to read out the reset level, and the difference between the signal level and the reset level is obtained. In the rest operation, the voltage of the FD section is set to the reset voltage Vdd, the noise is fixed to a noise ΔVn, which is different from the former ΔVn′ by random noise.
Accordingly, the reset level becomes Vdd+ΔVn, and the final output Vout becomes as follows.
                    Vout        =                ⁢                              (                          Vdd              +                              Δ                ⁢                                                                  ⁢                                  Vn                  ′                                            +              Vsig                        )                    -                                                ⁢                  (                      Vdd            +                          Δ              ⁢                                                          ⁢              Vn                                )                                        =                ⁢                  Vsig          +                      (                                          Δ                ⁢                                                                  ⁢                                  Vn                  ′                                            -                              Δ                ⁢                                                                  ⁢                Vn                                      )                              
That is to say, it is possible to eliminate an offset component, the voltage Vdd, but it is difficult to eliminate random noise, namely the noise ΔVn and the noise ΔVn′. In addition to thermal noise, the reset voltage Vdd fluctuates by the surrounding circuit operations as the power-source noise, and the fluctuations result in image-quality deterioration, such as unevenness on the screen (luminance unevenness on the screen), etc.